1. Field of the Invention
The invention relates to an image forming apparatus using an electrophotographic printing method or an electrostatic recording method, and particularly to an image forming apparatus such as a copying machine, a printer or a facsimile apparatus.
2. Description of the Related Art
An example of a conventional image forming apparatus will hereinafter be described with reference to FIGS. 8 and 9 of the accompanying drawings.
As shown in FIG. 8, a copying machine 801 as an image forming apparatus applies a laser beam to the surface of an image bearing member 803 uniformly charged by a charging device 802, in accordance with image information to form an electrostatic latent image.
The electrostatic latent image is developed into a toner image by a developing apparatus 804, whereafter the toner image on the image bearing member 803 is transferred onto a recording material S by a transfer device 805. The unfixed toner image on the recording material S in this state is heated and pressure-fixed by a fixing device 806 to obtain a fixed image.
The fixing device 806, as shown in FIG. 9, has a fixing roller 808 provided with a halogen heater 807 therein, a pressure roller 809 disposed in opposed relationship with the fixing roller 808, a cleaning web 810 for cleaning the fixing roller 808, etc.
The fixing roller 808 comprises of a hollow mandrel of aluminum having a thickness of 12 mm and a silicon rubber layer of the order of 100 μm as an intermediate layer provided thereon, and covered with fluorine resin such as PTFE (polytetrafluoroethylene) of the order of 20 μm so as to form a surface layer.
The pressure roller 809 comprises a mandrel of stainless steel and a silicone rubber layer having a thickness of 5 mm provided thereon, and covered with a tube of PFA (tetrafluoroethylene perfluoroalkylvinyl ether copolymer resin) of the order of 100 μm so as to form a surface layer.
The temperature of the fixing roller 808 is controlled so as to be maintained at 200 degrees by a control device 812. The unfixed toner 813 on the recording material S is fixed in the fixing area (fixing nip) between the fixing roller 808 and the pressure roller 809, and is permanently fixed on the recording material S.
The fixing roller 808 provided with the heater therein as described above adopts a silicone rubber layer having a small thickness in order to efficiently receive heat from the halogen heater 807 in the interior of the mandrel, and suppress the inconvenience that the silicone rubber layer is stripped off from the mandrel. Also, the pressure roller 809 adopts a silicone rubber layer thicker than that of the fixing roller 808 in order to increase the area per unit time in which the recording material S passes through the fixing nip. Further, the rubber layer of the fixing roller can be made thin to thereby shorten the time required to raise the temperature of the fixing roller to a fixing temperature, i.e., the so-called warmup time.
The outer diameter of each of the fixing roller 808 and the pressure roller 809 is 60 mm. The cleaning web 810 is formed of nonwoven fabric of polyamide or the like, and is impregnated with silicone oil. The cleaning web 810 is brought into contact with the surface of the fixing roller 808, and is taken up in a direction opposite to the direction of the rotation of the fixing roller 808 at a speed of 0.05 mm/s only during image formation to thereby remove the toner offset on the fixing roller 808.
A thermistor 811 for detecting the temperature of the fixing roller 808 is brought into contact with an area of the surface of the fixing roller 808 which the recording material S passes. The output of the halogen heater 807 is controlled on the basis of the temperature detected by the thermistor 811.
The maximum output of the halogen heater 807 during image formation is 1500 W, and can heat and fix 60 sheets of recording materials S of e.g. A3 size per minute. The rotating speed of the fixing roller 808 is controlled to 500 mm/s.
However, in such a fixing device as shown in FIG. 9, when embossed paper having a high demand for the printing of a note of invitation, an invitation card, a catalog, a pamphlet, etc. is passed, there is a case where the rubber layer of the fixing roller does not follow the irregularities of the surface due to embossing. In that case, the surface of the fixing roller becomes incapable of directly contacting with and heating the surfaces of the concave groove portions of the embossed paper, and thus, the fixability of the toner transferred to the groove portions is reduced.
As the result, there arise a problem such as an image fault due to a reduction in the density of the groove portions, and the problem that the toner which cannot be completely fixed is offset on the fixing roller in a great deal, and cannot be completely remove by the cleaning web to thereby contaminate the next recording material or contaminate the fixing device itself.